The most distinctive feature of Ctenophorans is the ‘combs’ – groups of cilia which they use for swimming – they are the largest animals that swim by means of cilia. Like cnidarians, their bodies are made of a mass of jelly, with one layer of cells on the outside and another lining the internal cavity. In ctenophores, these layers are two cells deep, while in cnidarians they are only one cell deep. Ctenophores also have a decentralized nerve net rather than a brain. Genomic studies have suggested that the neurons of Ctenophora, which differ in many ways from other animal neurons, evolved independently from those of the other animals. Ctenophores may balance marine ecosystems by preventing an over-abundance of copepods from eating all the phytoplankton.